The common, prior art approach to providing steering assist is to utilize a belt driven auxiliary pump, which runs continuously, coupled to an open center valve gear. This long standing system is highly inefficient, in that it is sized to do its work at parking maneuvers, and therefore produces significant energy losses when at highway speeds.
To alleviate this problem an "on demand" prior art approach for an electro-hydraulic, vehicular power steering system was developed, which included an electric motor driven, hydraulic pump, which charged an accumulator against an electronic metering valve. A pressure switch maintained a set pressure range in the accumulator.
The electronic metering valve, which was controlled by a microprocessor, metered the flow of high pressure hydraulic fluid to a standard steering gear from the accumulator. The microprocessor determined the amount of assist required, using inputs of road speed, steering wheel velocity and acceleration.
Such systems have functioned, but typically, not well. Prior attempts to improve the performance of such systems have helped, but only slightly.
An analysis of the prior system by applicant revealed the major problem with the system. The majority of the accumulator charge was being lost back to the reservoir through the standard open center steering gear without doing any work. To eliminate this energy loss, the valve timing in the gear would have to be modified. In addition, the timing of the metering valve and the steering gear valve would be extremely critical.
Further study of this prior system by applicant indicated that a better approach was to develop closed center valving for the steering gear. Incorporation of such a design change prohibits energy losses and potentially simplifies the system by eliminating the need for the metering valve, microprocessor and input sensors.
Those of significant skill in the vehicular art, including personnel of one of the "Big 3" manufacturers, were very skeptical of the operability of a closed center gear system concept, showing little interest. It is currently believed that such a system in one form or another had been attempted in the 1970s, with little or no success, and was abandoned.